The present invention relates to a semiconductor device, and more particularly to a high power and high breakdown voltage semiconductor device having a stable blocking characteristic.
A thyristor is a semiconductor device having three or more PN junctions and being capable of being switched from a current blocking state to a current conducting state by electrical or optical trigger means and vice versa by electrical means.
One of the typical examples thereof is explained with reference to a drawing. A PNPN type thyristor having an N-conductivity type semiconductor wafer as a starting material and manufactured in accordance with a conventional manufacturing process will be explained.
Referring to FIG. 1 a semiconductor substrate 10 has a P-type emitter (P.sub.E) layer 1 exposed to one major surface 101, an N-type base (N.sub.B) layer 2 adjacent to the P-type emitter layer 1, and an N-type emitter (N.sub.E) layer 4 adjacent to the N-type base layer 2 and exposed to the other main surface 102 of the semiconductor body 10 together with a P-type base (P.sub.B) layer 3. Formed between the P-type emitter layer 1 and the N-type base layer 2, between the N-type base layer 2 and the P-type base layer 3, and between the P-type base layer 3 and the N-type emitter layer 4 are PN junctions J.sub.1, J.sub.2 and J.sub.3, respectively, with the PN junctions J.sub.1 and J.sub.2 terminating at a side 103 of the semiconductor substrate 10 and the PN junction J.sub.3 terminating at the other major surface 102. An anode electrode 5, a cathode electrode 6 which are main electrodes and a control electrode 7 are formed on the one major surface 101 and at the exposed portions of the P-type base layer 3 of the other major surface 102 of the semiconductor substrate 10, respectively. The anode electrode also serves to protect the brittle semiconductor wafer. The PN junction J.sub.3 between the N-type emitter layer 4 and the P-type base layer 3 is partially shorted by the cathode electrode 6 at a region 41 to form a shorted emitter structure. The outermost periphery of the cathode electrode 6 is shorted by the P-type base layer 3 to form a shorted periphery structure 42. Accordingly, an end area 300 of the semiconductor substrate 10 has a PNP structure.
The shorted emitter structure and the shorted periphery structure are known techniques to improve the blocking characteristic of the thyristor. The blocking characteristic of the thyristor is defined as an ability to withstand as high voltage as possible with as small leaking current as possible when the voltage is applied across the anode electrode 5 and the cathode electrode 6 to reverse bias the junction J.sub.1 or J.sub.2 (i.e. blocking state). Usually, a high voltage can be blocked within the semiconductor body but the blocking ability is lower on the surface than in the inside because an electric field strength is higher on the surface than in the inside and hence an avalanche breakdown takes place on the surface. In order to avoid the above problem, it is necessary to establish a lower surface electric field strength than that in the inside. The reduction of the surface electric field strength can be attained by expanding a depletion layer on the surface.
For this purpose, in the prior art, the side edge 103 of the semiconductor substrate 10 has been shaped into a double bevel structure or sigma (.SIGMA.) contour. In this case, however, since the junctions J.sub.1 and J.sub.2 are exposed to the side edge 103, a surface passivation layer 200 has to be applied to prevent the reduction of the breakdown voltage due to the contamination and the deposition of impurity ions from the exterior.
For the semiconductor device having the side edge of the semiconductor substrate 10 shaped into the bevel structure and the passivation material applied on the side edge, the following technique has been proposed to render the breakdown voltage on the surface of the semiconductor substrate higher than a breakdown voltage of a bulk. Namely, in U.S. Pat. No. 3,413,527 to R. L. Davies issued on Nov. 26, 1968, it is proposed to provide a conductive guard electrode on a dielectric material in a thyristor having a side edge shaped into a bevel structure and the dielectric material deposited on the side edge, in the proximity of a PN junction in a semiconductor substrate. According to Davies, the conductive guard electrode serves to reduce an electric field strength on the side edge of the semiconductor body when the PN junction of the semiconductor substrate is reverse biased to render the breakdown voltage on the surface higher than the breakdown voltage of the bulk.
However, the prior art thyristor has the following problem. In the thyristor, when a high blocking voltage is applied for an extended time period, a leak current increases abnormally so that the blocking characteristic is materially degraded, and in the worst case, a thermal run-away takes place to break the device.
U.S. Pat. No. 3,413,527 does not refer to the problem and the solution therefor in applying the blocking voltage for the extended time period.
For this problem, it has been generally accepted to consider that it is not due to a phenomenon in the semiconductor body but due to the side edge of the semiconductor substrate 10 in connection with the passivation material. Accordingly, the passivation material per se and the chemical process for the side edge have been studied.
However, a specific model for the causes of the degradation and a solution therefor have not been studied on a structure close to an actual device as shown in FIG. 1, and no semiconductor device has been proposed which resolved the above problem from a standpoint of structure apart from the standpoint of the passivation material and the manufacturing process.